Semicarbazide-sensitive amine oxidase (SSAO) activity is an enzyme activity expressed by Vascular Adhesion Protein-1 (VAP-1) or Amine Oxidase, Copper Containing 3 (AOC3), belongs to the copper-containing amine oxidase family of enzymes (EC.1.4.3.6). Therefore inhibitors of the SSAO enzyme may also modulate the biological functions of the VAP-1 protein.
SSAO activity has been found in a variety of tissues including vascular and non-vascular smooth muscle tissue, endothelium, and adipose tissue [Lewinsohn, Braz. J. Med. Biol. Res. 1984, 17, 223-256; Nakos & Gossrau, Folia Histochem. Cytobiol. 1994, 32, 3-10; Yu et al., Biochem. Pharmacol. 1994, 47, 1055-1059; Castillo et al., Neurochem. Int 1998, 33, 415-423; Lyles & Pino, J. Neural. Transm. Suppl. 1998, 52, 239-250; Jaakkola et al., Am. J. Pathol. 1999, 155, 1953-1965; Morin et al., J. Pharmacol. Exp. Ther. 2001, 297, 563-572; Salmi & Jalkanen, Trends Immunot 2001, 22, 211-216]. In addition, SSAO protein is found in blood plasma and this soluble form appears to have similar properties as the tissue-bound form [Yu et al., Biochem. Pharmacol. 1994, 47, 1055-1059; Kurkijärvi et al., J. Immunol. 1998, 161, 1549-1557].
The precise physiological role of this abundant enzyme has yet to be fully determined, but it appears that SSAO and its reaction products may have several functions in cell signalling and regulation. For example, recent findings suggest that SSAO plays a role in both GLUT4-mediated glucose uptake [Enrique-Tarancon et al., J. Biol. Chem. 1998, 273, 8025-8032; Morin et al., J. Pharmacol. Exp. Ther. 2001, 297, 563-572] and adipocyte differentiation [Fontana et al., Biochem. J. 2001, 356, 769-777; Mercier et al., Biochem. J. 2001, 358, 335-342]. In addition, SSAO has been shown to be involved in inflammatory processes where it acts as an adhesion protein for leukocytes [Salmi & Jalkanen, Trends Immunot 2001, 22, 211-216; Salmi & Jalkanen, in “Adhesion Molecules: Functions and Inhibition” K. Ley (Ed.), 2007, pp. 237-251], and might also play a role in connective tissue matrix development and maintenance [Langford et al., Cardiovasc. Toxicol. 2002, 2(2), 141-150; Göktürk et al., Am. J. Pathol. 2003, 163(5), 1921-1928]. Moreover, a link between SSAO and angiogenesis has recently been discovered [Noda et al., FASEB J. 2008, 22(8), 2928-2935], and based on this link it is expected that inhibitors of SSAO have an anti-angiogenic effect.
Several studies in humans have demonstrated that SSAO activity in blood plasma is elevated in conditions such as congestive heart failure, diabetes mellitus, Alzheimer's disease, and inflammation [Lewinsohn, Braz. J. Med. Biol. Res. 1984, 17, 223-256; Boomsma et al., Cardiovasc. Res. 1997, 33, 387-391; Ekblom, Pharmacol. Res. 1998, 37, 87-92; Kurkijärvi et al., J. Immunol. 1998, 161, 1549-1557; Boomsma et al., Diabetologia 1999, 42, 233-237; Meszaros et al., Eur. J. Drug Metab. Pharmacokinet. 1999, 24, 299-302; Yu et al., Biochim. Biophys. Acta 2003, 1647(1-2), 193-199; Mátyus et al., Curr. Med. Chem. 2004, 11(10), 1285-1298; O'Sullivan et al., Neurotoxicology 2004, 25(1-2), 303-315; del Mar Hernandez et al., Neurosci. Lett. 2005, 384(1-2), 183-187]. It has been suggested that reactive aldehydes and hydrogen peroxide produced by endogenous amine oxidases contribute to the progression of cardiovascular diseases, diabetic complications and Alzheimer's disease [Callingham et al., Prog. Brain Res. 1995, 106, 305-321; Ekblom, Pharmacol. Res. 1998, 37, 87-92; Yu et al., Biochim. Biophys. Acta 2003, 1647(1-2), 193-199; Jiang et al., Neuropathol Appl Neurobiol. 2008, 34(2), 194-204]. Furthermore, the enzymatic activity of SSAO is involved in the leukocyte extravasation process at sites of inflammation where SSAO has been shown to be strongly expressed on the vascular endothelium [Salmi et al., Immunity 2001, 14(3), 265-276; Salmi & Jalkanen, in “Adhesion Molecules: Functions and Inhibition” K. Ley (Ed.), 2007, pp. 237-251]. Accordingly, inhibition of SSAO has been suggested to have a therapeutic value in the prevention of diabetic complications and in inflammatory diseases [Ekblom, Pharmacol. Res. 1998, 37, 87-92; Salmi et al., Immunity 2001, 14(3), 265-276; Salter-Cid et al., J. Pharmacol. Exp. Ther. 2005, 315(2), 553-562].
WO2007146188 teaches that blocking SSAO activity inhibits leucocyte recruitment, reduces the inflammatory response, and is expected to be beneficial in prevention and treatment of seizures, for example, in epilepsy.
O'Rourke et al (J Neural Transm. 2007; 114(6):845-9) examined the potential of SSAO inhibitors in neurological diseases, having previously demonstrated the efficacy of SSAO inhibition in a rat model of stroke. An SSAO inhibitor is tested on relapsing-remitting experimental autoimmune encephalomyelitis (EAE), a mouse model that shares many characteristics with human multiple sclerosis. The data demonstrates the potential clinical benefit of small molecule anti-SSAO therapy in this model and therefore in treatment of human multiple sclerosis.
SSAO knockout animals are phenotypically overtly normal but exhibit a marked decrease in the inflammatory responses evoked in response to various inflammatory stimuli [Stolen et al., Immunity 2005, 22(1), 105-115]. In addition, antagonism of its function in wild type animals in multiple animal models of human disease (e.g. carrageenan-induced paw inflammation, oxazolone-induced colitis, lipopolysaccharide-induced lung inflammation, collagen-induced arthritis, endotoxin-induced uveitis) by the use of antibodies and/or small molecules has been shown to be protective in decreasing the leukocyte infiltration, reducing the severity of the disease phenotype and reducing levels of inflammatory cytokines and chemokines [Kirton et al., Eur. J. ImmunoL 2005, 35(11), 3119-3130; Salter-Cid et al., J. Pharmacol. Exp. Ther. 2005, 315(2), 553-562; McDonald et al., Annual Reports in Medicinal Chemistry 2007, 42, 229-243; Salmi & Jalkanen, in “Adhesion Molecules: Functions and Inhibition” K. Ley (Ed.), 2007, pp. 237-251; Noda et al., FASEB J. 2008 22(4), 1094-1103; Noda et al., FASEB J. 2008, 22(8), 2928-2935]. This anti-inflammatory protection seems to be afforded across a wide range of inflammatory models all with independent causative mechanisms, rather than being restricted to one particular disease or disease model. This would suggest that SSAO may be a key nodal point for the regulation of the inflammatory response, and it is therefore likely that SSAO inhibitors will be effective anti-inflammatory drugs in a wide range of human and animal diseases. VAP-1 has also been implicated in the progression and maintenance of fibrotic diseases including those of the liver and lung. Weston and Adams (J Neural Transm. 2011, 118(7), 1055-64) have summarized the experimental data implicating VAP-1 in liver fibrosis, and Weston et al (EASL Poster 2010) reported that blockade of VAP-1 accelerated the resolution of carbon tetrachloride induced fibrosis. In addition VAP-1 has been implicated in inflammation of the lung (e.g. Singh et al., 2003, Virchows Arch 442:491-495) suggesting that VAP-1 blockers would reduce lung inflammation and thus be of benefit to the treatment of cystic fibrosis by treating both the pro-fibrotic and pro-inflammatory aspects of the disease.
SSAO (VAP-1) is up regulated in gastric cancer and has been identified in the tumour vasculature of human melanoma, hepatoma and head and neck tumours (Yoong K F, McNab G, Hubscher S G, Adams D H. (1998), J Immunol 160, 3978-88.; lrjala H, Salmi M, Alanen K, Gre'nman R, Jalkanen S (2001), Immunol. 166, 6937-6943; Forster-Horvath C, Dome B, Paku S, et al. (2004), Melanoma Res. 14, 135-40.). One report (Marttila-lchihara F, Castermans K, Auvinen K, Oude Egbrink M G, Jalkanen S, Griffioen A W, Salmi M. (2010), J Immunol. 184, 3164-3173.) has shown that mice bearing enzymically inactive VAP-1 grow melanomas more slowly, and have reduced tumour blood vessel number and diameter. The reduced growth of these tumours was also reflected in the reduced (by 60-70%) infiltration of myeloid suppressor cells. Encouragingly VAP-1 deficiency had no effect on vessel or lymph formation in normal tissue.
For the above reasons, it is expected that inhibition of SSAO will reduce the levels of pro-inflammatory enzyme products (aldehydes, hydrogen peroxide and ammonia) whilst also decreasing the adhesive capacity of immune cells and correspondingly their activation and final extra-vasation. Diseases where such an activity is expected to be therapeutically beneficial include all diseases where immune cells play a prominent role in the initiation, maintenance or resolution of the pathology, such inflammatory diseases and immune/autoimmune diseases. Examples of such diseases include multiple sclerosis, arthritis and vasculitis.
An unmet medical need exists for new inhibitors of SSAO having utility in medicine, including veterinary medicine, in the treatment of humans and animals suffering from a condition susceptible to modulation by an SSAO inhibitor.